1. Field of the Invention
The present invention relates to an on-screen input image display device, in particular, an on-screen input image display system in which an electrostatic capacity coupling method is employed and uniform resolution is realized over a display screen.
2. Description of the Related Art
An image display device including a touch sensor (also referred to as touch panel) having an on-screen input function for inputting information to a display screen by a touch operation with a user's finger or the like (contact and press operation, hereinafter, simply referred to as “touch”) is used for a mobile electronic device such as a PDA or a cellular phone, various home electric appliances, a stationary customer guiding terminal such as an automatic reception machine, and the like. As to the image display device having such a touch input function, there are some known methods including a method of detecting a change in resistance value or capacitance of a touched part, a method of detecting a change in quantity of light at the part shielded by the touch operation, and the like.
Of those, in an electrostatic capacity coupling method, a rear surface ITO in a display panel constituting a liquid crystal display device of in-plane switching (IPS) method can be used as a detection transparent conductive film for detecting an electrostatic capacity generated by a touch of a user's finger. Thus, this method is a promising candidate for a technology which realizes a touch panel at low cost.
An on-screen input display device of electrostatic capacity coupling method is disclosed in JP2006-146895 A. In JP2006-146895 A, for the purpose of implementing screen input, electrode terminals are provided at four corners of a detection transparent conductive film, and then an AC signal is applied through a current detection resistor. A parasitic capacitance and an electrostatic capacity (generated by a touch of a finger) in the detection transparent conductive film are charged and discharged with the AC signal. At that time, currents that flow through the individual electrode terminals at the four corners are determined, and then coordinate values are calculated from current ratios thereof. In this current detection, in order to reduce a noise caused by a driving signal for the display panel, a strobe signal is generated based on a liquid crystal driving timing, and the period under the noise influence is masked using this strobe signal. This permits noise reduction and improves detection accuracy.